From the earliest of times human beings have been concerned with their appearance and the condition of their external features, their skin, hair, fingernails, etc. Accordingly, a myriad of cosmetic compositions have been developed to satisfy the demand for such products. Cosmetics often are designed to have both utilitarian and aesthetic appeal. Although the utilitarian aspects of the compositions of this invention have not been neglected, it is primarily with the aesthetic aspects that this invention is concerned. In this regard, the cosmetic compositions of this invention exhibit a beautiful and intriguing phenomenon known as "structural color."
Briefly, when two transparent, immiscible liquids are mixed, the combination is often cloudy. If, however, the liquids have the same refractive index (generally measured at 589 nm, i.e., the sodium D line, and 20.degree. C., viz., "n.sub.D.sup.20 "), the mixture will be substantially transparent to the human eye and appear to be homogeneous. The appearance of "structural color" in such a mixture requires, not only that the refractive indexes (at a given wavelength of visible light) are the same, but that the variation of the indexes as a function of visible wavelength differ for the two liquids. That is, the "dispersive power" of the two phases must be different.
Chemical systems which exhibit structural color have been referred to as "chromatic emulsions" by Holmes and Cameron, J. Am. Chem. Soc., 44, pp 71-74 (1922), who traced the first report of the phenomenon to 1913. Indeed the liquid mixtures may contain emulsifying agents, and the mixtures may be transparent emulsions. Emulsions are especially useful in cosmetic applications; see, e.g., Cosmetics & Toiletries. 101, pp 25-44 (1986). However, emulsification is not a requirement.
Holmes and Cameron provided the following illustrations: When glycerol (n.sub.D.sup.20 =1.4660) and amyl acetate (n.sub.D.sup.20 =1.4012) were shaken together, a milky-white mixture resulted, a typical emulsion. However, when water (n.sub.D.sup.20 =1.3330) was added to the glycerol until n.sub.D.sup.20 of that solution was 1.4012, and amyl acetate was shaken with the solution, a perfectly transparent emulsion was obtained; however, no color is reported. Further, the authors report: "In attempting to disperse glycerol in an acetone solution of cellulose nitrate we failed to get transparency. Since the index of refraction of the acetone (1.35886) was lower than that of the glycerol (1.4660) we added benzene (1.50144) cautiously to the milky emulsion in order to equalize the indices of the two liquid phases of course the benzene diluted the acetone thus becoming part of the continuous phase. With cautious additions, and shaking, increased transparency was secured but accompanied by a startling development of colors. At first the emulsion became yellow as viewed from the side and a soft blue when held between the eye and the source of light. With further addition of benzene the yellow changed to beautiful pink while the blue became green. More benzene changed the pink to lavender and later to a peacock-blue. Finally, the emulsion lost color and became milky. The colors are restored, in reverse order, by cautious additions of acetone."
Structural color as exhibited in chromatic emulsions has been described subsequent to the paper of Holmes and Cameron. Such reports appear in J. Phys. Chem., 56, pp 510-513 (1952) and in P. Becher, "Emulsions: Theory and Practice," 2nd Ed., Reinhold Publishing Corp., New York, N.Y., 1965, p 58, for example. The systems described in this prior art were not directed toward cosmetic applications, and the need to use cosmetically acceptable components was not addressed; rather, aromatic hydrocarbons (e.g., benzene), halocarbons (e.g., chloroform and carbon tetrachloride), and amines (e.g., pyridine) were mentioned as components.
Transparent emulsions which might have application in cosmetics are disclosed in Int'L. J. Cosmet. Sci., 8, pp 1-8 (1986) and in UK published application 2 079 300 A, for example. Various cosmetically acceptable components are cited, including silicone compounds. However, none of the cosmetic compositions is said to exhibit structural color, so the requirement of a difference in dispersive power between the two phases apparently was neither sought nor achieved.
To be distinguished from the emulsions of this invention, which exhibit structural color, are transparent microemulsions in which the transparency is achieved, not by virtue of refractive index matching, but rather by controlling the diameter of the dispersed phase particles, making them so small, less than about 0.05 micron, that they cannot be resolved by visible light. The compositions of this invention are not restricted to emulsions; but to the extent the compositions are emulsions, they can, if desired, be macroemulsions, no attempt being required to control the dispersed phase particle size.
Consequently, it is one object of this invention to provide useful cosmetic compositions with the aesthetic feature of exhibiting structural color. It is another objective to provide such compositions using cosmetically acceptable components.